In order to read or inspect an object which is imaged by an image forming optical system, it is necessary to illuminate the object. An illuminating optical system is typically constructed as a separate system from the image forming optical system; however, by employing a structure which introduces illuminating light at an intermediate point of the image forming optical system, the overall size of the apparatus can be reduced. Referring to FIG. 7, illuminating light 70 is introduced at an intermediate point of an image forming optical system from a light source 72. The illuminating light 70 is reflected by a reflecting surface (RS), which is a partially-reflecting surface that allows some light to pass through and reflects some light. The reflected light 74 passes through a first lens group (Gr1) and a filter (FL), and a portion of the reflected light 74 (ordinary light 76) is reflected by an object (not shown). The ordinary light 76 passes again through the filter (FL) and the first lens group (Gr1). The ordinary light 76 then passes through the reflecting surface (RS), a second lens group (Gr2), and falls on an imaging surface (IS) as an image 78.
As can be seen in FIG. 8, some of the reflected light 74 is reflected as an extraordinary component 82 from one or more lens surfaces 80 and reaches the imaging surface (IS) as a ghost 84 before the reflected light 74 reaches the object to be imaged. This situation causes ghosts in the image, which are typically detrimental. The ghosting effect is particularly great when the reflectance of the object being imaged is low.